The present invention relates to apparatus for nuclear magnetic resonance and is particularly concerned with improvements in signal-to-noise ratio associated therewith.
Nuclear magnetic resonance experiments are inevitably limited by the signal to noise achievable in the experimental configuration. FIG. 1 shows a schematic representation of an idealized NMR experiment. A nuclear spin having a magnetic moment is represented by an arrow. The dipolar field B1 associated with this spin is represented by several flux lines. The spin is located at the center of a spherical NMR sample. The sample and the spins contained within it are immersed in a homogeneous magnetic field B0, which points in the z direction. A coordinate system defines the plane of the drawing to contain the x and y coordinates. When the spin is excited into the xy plane by a radio frequency (RF) pulse whose magnetic component parallels the y-axis, it precesses around its own center with a precession frequency given by xcfx89, which is proportional to the magnetic field. xcfx89 is known as the Larmor frequency and is given by xcfx89=xcex3B0, where xcex3 is the gyromagnetic constant for a given isotope and B0 is the static magnetic field strength. For hydrogen with xcex3=42.58 MHzxcexxe2x88x921 with a B0 of 1.0 Tesla the precession frequency is 42.58 MHz. Around the sample is placed a receiver coil, which is circular in shape in the xz plane. The receiver coil is connected to tuned and matched electronic circuits in the manner known in the art of nuclear magnetic resonance.
The sensitivity of a NMR experiment can be appreciated from the observation that many of the dipolar B1 flux lines never intersect the receiving coil as the spin rotates within the sample. For voltage to be induced in the receiver coil, flux lines must intersect the coil as the spin rotates. Only a few of the peripheral flux lines intersect with the receiver coil in this manner. The situation could be improved by using a smaller coil closer to the spin, but the coil needs to remain outside the sample volume.
The signal-to-noise ration (SNR) is determined by the voltage induced in the receiving coils by the rotating dipole divided by the thermionic noise voltage in the receiving coil. In the particular case of large samples, the noise arises primarily from thermally generated magnetic noise within the sample. This is the situation in most human magnetic resonance imaging. However for higher frequency (greater than several hundred megahertz) NMR experiments with small samples, the noise is invariably primarily from the coil and electronics.
One way to increase the sensitivity of the receiving coil is to increase the amount of B1 field intersecting the receiver coil during one revolution of the dipole. This can be accomplished by providing a high magnetic permeability return path for the B1 emanating from the dipole.
In accordance with an aspect of the present invention more is provided an apparatus for nuclear magnetic resonance (NMR) comprising a sample container for a chemical sample; a receiver coil adjacent to the sample container, and a flux return yoke for guiding flux lines around the outside of the receiver coil causing the number of the flux lines intersecting with the receiver coil to be increased so as to increase a signal to noise ratio of an NMR experiment. The return yoke includes a material having a particular magnet permativity tuned for a particular frequency range, e.g. the Larmor frequency.
Advantages of the present invention include allowing equivalent SNR experiments to be executed with lower strength field magnets/and improving SNR with same level of magnetic field.